It is proposed to use Drosophila photoreceptor-potential mutants to identify the intermediate steps in phototransduction and thereby to unravel the molecular mechanism of visual excitation. To this effect biochemical studies, guided by electrophysiological experiments in intact cells, will be carried out on Drosophila mutants. A large repetoire of these mutants already exist but have never been used for this purpose. Sensitive and specific assays of light dependent GTPase, cyclic GMP phosphodiesterase, protein phosphorylation and protein dephosphorylation will be designed. The relevance of the light-activated reactions to phototransduction will be established by measuring enzyme activities on conditional phototransduction defective mutants. Genetic dissection has established that mutation in five different genes can each cause photoreceptor cell degeneration. At least one of these (rdgB) can be cured by the simultaneous presence of norpA gene. Elucidation of the molecular mechanism of the affected gene product can be of great value in understanding the etiology of retinal degeneration and can indicate possible methods of therapy. We have already succeeded in measuring light dependent GTPase activity in particulate membrane preparation from Drosophila. This assay will be used to purify the rhodopsin containing membrane and other enzymes will be assayed on this preparation. The hypothesis that the prolonged depolarizing after potential (PDA) and light coincident receptor potential (LCRP) are generated by similar voltage fluctuations will be tested by shot noise analysis. The characteristics of Na-Ca exchange mechanisms will be studied and mutants which have defects in this mechanism will be searched for. Evidence exists that the PDA in flies is similar to an afterpotential induced by bleaching adaptation in vertebrate rods and that the PDA results upon photoconversion of neighboring rhodopsin molecules. Phsophorylation of metarhodopsin in membrane preparations will be examined as a mechanism of metarhodopsin inactivation. Positive results will enable us to test whether the PDA is induced by an inactivation resistant complex of metarnodopsin molecules. This experiment may lead to a molecular explanation for this "memory like Phenomenon".